The patients treated with hydroxychloroquine or the related drug chloroquine were at higher risk of developing arrhythmia — an irregular heart rhythm that if severe and/or combined with existing cardiac problems, can lead to sudden death.

Published Friday in the medical journal The Lancet, the report comes with some caveats: It is based on a review of medical records, not a controlled study with a randomly selected control group.

Researchers analyzed data from 671 hospitals on six continents, involving just over 96,000 patients hospitalized from Dec. 20 to April 14 who tested positive for the coronavirus. About 15,000 received some form of hydroxychloroquine or chloroquine.

“We did not observe any benefit of hydroxychloroquine or chloroquine (when used alone or in combination with a macrolide) on in-hospital outcomes, when initiated early after diagnosis of COVID-19,” the researchers concluded. “Each of the drug regimens of chloroquine or hydroxychloroquine alone or in combination with a macrolide was associated with an increased hazard for clinically significant occurrence of ventricular arrhythmias and increased risk of in-hospital death with COVID-19.”

Both the Food and Drug Administration and the World Health Organization have warned against using the drug, which is typically prescribed for malaria and autoimmune diseases, to treat COVID-19.